Modern RADAR system architectures include analog and digital phased array systems. A phased array antenna is comprised of numerous radiating elements each having a phase shifter. Beams are formed by selectively activating all or a portion of antenna elements of a given array. The beam pattern of these antennas can be controlled to produce one or more directed beams. Scanning or steering of the beams is accomplished by shifting the phase of the signals emitted from the elements in order to provide constructive and/or destructive interference. The ability to form and steer a beam permits multiple functions to be performed by the same system. As with any type of RADAR, the ability to efficiently test the functionality and accuracy of the system may be critical to achieving and maintaining system performance.
Phased RADAR arrays using analog beamforming have traditionally been tested using a planar nearfield scanner. More specifically, radio frequency (RF) equipment is configured to either receive an RF signal from an analog Array Under Test (AUT), such as AUT 10 in FIG. 1A, or transmit an RF signal to an AUT. In order to obtain the phase and amplitude changes for determining AUT performance, the equipment is configured to establish reference points at a nearfield probe and at the AUT. During nearfield scanning measurements, data is acquired via an RF receiver, and amplitude and phase measurements are characterized by the difference between the reference and test points. After the entire active aperture has been scanned and corresponding data collected, the data is processed via a Fourier Transform to obtain a pattern of the farfield energy in visible space.
As phased array technology has improved, universal digital beam forming (DBF) architecture has been implemented. Digital receivers and exciters have greatly improved phased array performance parameters, such as signal to noise, beamforming error elimination, and clutter attenuation. Many new digital phased array architectures are designed utilizing distributed digital receivers and exciters (DREX). DREX systems include, for example, an “on-array” module having a single exciter, a single receiver and a transmit/receive module (T/R module) associated with each radiating element. DREX systems offer improved RADAR performance in detection sensitivity, time sidelobes, third-order interception point and instantaneous dynamic change. However, the implementation of these on-array exciters and receivers poses new technical challenges.
For example, new testing and measurement obstacles have been encountered, as the array can no longer be represented as a transfer function of a linear system. More specifically, analog phased arrays can be represented as a complex transfer function dependent on the carrier frequency. The transfer function H(jω) is simply the ratio of RFin to RFout:H(jω)=RFout/RFin 
This linear function is relatively easily tested using a standard network analyzer. The analyzer measures the swept frequency magnitude and phase difference induced by the transfer function relative to the analyzer's self-created reference signal of known magnitude and phase.
As set forth above, DREX systems used in digital phased arrays can no longer be represented by an input RF signal, a transfer function and an RF output signal. Modern digital distributed RADAR arrays, as illustrated by digital AUT 15 in FIG. 1B, convert a digital bit stream to an RF signal when operating in a transmit (Tx) mode, and convert a received RF signal to a stream of digital bits when operating in a receive (Rx) mode. As a result of this AD/DA processing, RF-based test equipment (i.e. network analyzers) are not adequate to accurately test, debug or characterize these systems.
More specifically, when a digital array transmits to the nearfield scanner probe, a digital word is translated by the exciter's D/A converter and transmitted as an RF signal to the nearfield scanner probe. Conversely, the array receives an RF signal from the nearfield scanner probe, and an A/D converter on the digital receiver converts the analog data to digital data comprising in-phase and quadrature-phase (I/O) data components. This I/Q data stream is then distributed to various processing locations within the phased array system.
As the nearfield scanner probe uses an analog receiver and exciter, existing systems must correlate digital I/Q data from the digital array with the analog RF data of the nearfield scanner probe. For example, the DREX modules in the array require the distribution of multiple local oscillators for up-conversion in Tx mode and down-conversion in Rx mode. Variations in magnitude and phase of each local oscillator are wrongly perceived as changes in amplitude or phase of the RADAR target return signal. Therefore, care must be taken in the distribution of the RADAR RF circuitry (for Tx and Rx) in the array, as well as the distribution of the local oscillator signals in the array. This same care must be applied to the external equipment testing or characterizing the array, or the constituent components of the array. Further, the reference RF signal used for comparison to detect amplitude and phase changes is problematic. If the reference RF signal exhibits parameter changes, the array will wrongly apply internal corrections as though the array hardware was exhibiting errors.
Current solutions for testing DREX-based systems include the application of identical DREX hardware used as the testing equipment. Outfitting this RADAR-testing-RADAR solution means adding significant amounts of additional hardware, software, firmware, data recording/storage ability, and RF and local oscillator stability equipment to the system. These systems are not portable for testing or alignment of the array in the field. In addition to the added weight and expense, the system is further taxed with the need for additional cooling for the hardware. Engineering of this arrangement is also intensive and expensive.
Improved systems and methods for nearfield testing of phased arrays are desired.